A drug used to help Vietnam veterans overcome the horrors of war is set to be marketed in Britain as a 'miracle' cure for smoking, after researchers discovered that it staves off nicotine cravings.

The drug, which will be available on prescription as Zyban and released here next year, acts on the central nervous system to reduce withdrawal symptoms.

Nearly three million people have taken the product in the United States since it went on sale as a stop-smoking drug two years ago.

Clinical trials show that Zyban had almost double the success rate (people quitting for one year) of other therapies such as nicotine patches.

Rob Cohen, a spokesman for the manufacturer Glaxo Wellcome, said: 'We hope the product will be available in Britain next year. We are currently seeking a licence in Europe.'

Originally marketed in the early Eighties as an anti-depressant, Zyban never received a British licence because the then-government said it carried an unacceptable risk of seizures.

It was available, however, in the US where the company refined technology and developed a 'slow release' chemical formula which cut the seizure risk to the level of other anti-depressants, such as Prozac.

In 1993 a chance observation by a Californian psychiatrist, Dr Linda Ferry, found that the Vietnam veterans she was treating with the drug seemed less inclined to smoke.

She contacted Wellcome which at that time showed little interest in her findings. It was not until she carried out a pilot study testing her theory that the pharmaceutical company took note.

Professor Martin Jarvis of the Imperical Cancer Research Fund, a leading expert on smoking, said: 'It's clear from clinical studies that Zyban reduces a smoker's craving and is an effective way of helping people to stop, but how it works is a mystery.

'Nobody really understands its mode of action because it has nothing to do with its anti-depressant effect.

'The evidence shows that it reduces craving by taking away the reward value of nicotine. It would be wrong to say that no craving exists at all and smokers will still find it hard to give up.'

Smokers crave cigarettes because they are used to nicotine stimulating neuro-transmitters in their brain. Zyban somehow increases these brain chemicals, simulating the rush smokers get when they inhale cigarettes.

Taking Zyban can help even out the troughs of craving that a person trying to give up may experience.

Experts say that smokers have a 50 per cent chance of being killed by their habit and on average lose eight years of their life.

Jarvis said: 'Zyban is exciting because at last we have a non-nicotine drug that is helping scientists understand how nicotine addiction works.'

The clinical trial, published in the New England Journal of Medicine earlier this year, examined the quit rates of 900 smokers. Scientists found that 30 per cent using Zyban gave up after a 10-week course, compared with 15 per cent using nicotine replacement patches.

Clive Bates, director of Action on Smoking and Health (ASH), said: 'It is better than going cold turkey but people shouldn't underestimate how difficult it is to give up. This product helps with the physical side of the addiction but you still need willpower.'

According to the World Health Organinsation, three million people died last year from tobacco-related causes.

In Britain 13 million people smoke. Although 70 per cent say they want to give up, only 300,000 manage to each year.

Dean Mahoney, a spokesman for the Health Education Authority, said: 'In the last two years the nicotine replacement market has grown enormously. It is now worth £40 million and the most popular method is still the chewing gum.

'However, quit rates are still depressingly low, and the Government is committed to helping smokers stop. Anything that makes this easier is welcome.'

Rival pharmaceutical companies are currently looking at ways of using other chemical compounds to help people stop smoking.

Zyban costs about £3.60 for a day's supply in the US, but no details of prices for Britain have yet been released.

Seven ways to help you stub it out

Nicotine patch This is applied like a sticking plaster to dry non-hairy skin, for example the upper arm. It lasts either 16 or 24 hours and can be bought in three strengths. Patches cost around £15 a week and the recommended period of use is three months.

Nicotine chewing gum Should be chewed slowly to deliver a steady dose of nicotine over about 30 minutes. Available in two strengths and a variety of flavours. Cost £15 per week.